1. Field of the Invention
The present invention relates to a high-voltage power source which is applicable to an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
Conventionally, a copying machine, an inkjet printer, a laser beam printer and the like are known as an image forming apparatus which forms an image on a sheet. FIG. 11 illustrates a configuration of a laser beam printer which will be described below as an example of an image forming apparatus.
Referring to FIG. 11, in the laser beam printer, a photosensitive drum 101 is an electrostatic latent image carrier, and a semiconductor laser 102 is used as a light source for forming an electrostatic latent image on the photosensitive drum 101. A motor 104 rotates a rotational polygon mirror 103, and a laser beam 105 emitted from the semiconductor laser 102 scans the photosensitive drum 101. A charging roller 106 is a charging member which nearly uniformly charges a surface of the photosensitive drum 101. A developing unit 107 develops the electrostatic latent image formed on the photosensitive drum 101 using a toner as a developer. A transfer roller 108 is a transfer member for transferring a toner image developed by the developing unit 107 on a sheet. A fixing roller 109 serves as a fixing unit for fusing a toner image transferred on a sheet with heat and pressure. A process cartridge 100 in which the photosensitive drum 101, the charging roller 106, and the developing unit 107 are integrated is detachably mounted on the image forming apparatus.
A first feeding roller 110 rotates once to feed a sheet one by one from a cassette 127. The cassette 127 includes a function (not illustrated) for identifying a sheet size. A manual feeding roller 111 feeds a sheet to a conveyance path from a manual feed port (not illustrated) that does not include a function for identifying a sheet size. A second feeding roller 112 feeds a sheet to the conveyance path from a cassette 128 that is an optional feeding device detachably attached to the image forming apparatus. An envelope feeding roller 113 feeds one envelope at a time to the conveyance path, from an envelope feeder (not illustrated) that is detachably attached and can only stack envelopes. Conveyance rollers 114 and 115 convey a sheet that is fed from each of the cassettes 127 and 128.
A sheet detection sensor 116 detects a sheet which is fed from a source other than the envelope feeder, and a conveyance roller 117 feeds the conveyed sheet to the photosensitive drum 101. A sheet position detection sensor 118 synchronizes a leading position of the fed sheet with an image writing position (recording/printing) of the photosensitive drum 101. At the same time, the sheet position detection sensor 118 measures a length of the fed sheet in a conveying direction (by detecting a leading edge and a trailing edge). A sheet discharge sensor 119 detects whether there is a sheet after fixing an image, and a discharging roller 120 discharges a sheet on which an image is fixed to outside of the apparatus.
A flapper 121 switches a conveying destination of a printed sheet. The printed sheet can be conveyed to a discharge tray (not illustrated) on which the sheet is discharged in a face-down state (i.e., with a printed side facing downward) in an outside of the apparatus. The printed sheet can also be conveyed to a two-sided conveyance path 129 for reversing and conveying the sheet to form an image on both sides of the sheet.
A conveyance roller 122 conveys a sheet conveyed to the two-sided conveyance path 129 to a reversing unit (not illustrated), and a sensor 123 detects the sheet conveyed to the reversing unit. A reverse conveyance roller 124 reverses the sheet at a predetermined timing and feeds the sheet to the two-sided conveyance path 129. A sensor 125 detects the sheet at the two-sided conveyance path 129, and a conveyance roller 126 feeds the reversed sheet to the conveyance path for performing image formation again. The two-sided conveyance path 129, the conveyance roller 122, the reverse conveyance roller 124, the conveyance roller 126, and the sensor 125 are unitized as a two-sided conveyance unit 130 which is detachably attached to the image forming apparatus.
FIG. 12 illustrates a block diagram of a control circuit for controlling the image formation of the above-described image forming apparatus.
Referring to FIG. 12, a printer controller 201 includes a function for rasterizing code data of an image sent from an external device such as a host computer (not illustrated) into bit map data which is necessary for printing. The printer controller 201 reads information about an internal status of a printer (e.g., information about sheet conveyance status or whether there is sheet inside a cassette) and instructs and manages a printer operation based on the read information. Further, the printer controller 201 includes a function for displaying the read printer status.
An engine control unit 202 controls various units of a printer engine according to an instruction from the printer controller 201. The engine control unit 202 includes a function for notifying information about the internal status of the printer to the printer controller 201. A sheet conveyance control unit 203 drives and stops a driving unit (e.g., motor, not illustrated) of conveyance rollers for conveying a sheet according to an instruction from the engine control unit 202. A high-voltage control unit 204 controls high voltage output in a charging operation by a charging roller, a developing operation by a developing unit, and a transferring operation by a transfer roller respectively, according to an instruction from the engine control unit 202. An optical system control unit 205 controls the driving and stopping of the motor 104 and emission of a laser beam 105 according to an instruction from the engine control unit 202. A sensor input unit 206 inputs an output from sensors 116, 118, 119, 123 and 125. A fixing temperature control unit 207 adjusts a temperature of a fixing unit to a temperature designated by the engine control unit 202.
An option cassette control unit 208 controls an operation of a detachably attached option cassette. The option cassette control unit 208 drives and stops a driving system of the option cassette according to an instruction from the engine control unit 202 and sends information about whether there is sheet in the option cassette and sheet size.
A two-sided conveyance unit control unit 209 controls an operation of the two-sided conveyance unit 130 that is detachably attached to the image forming apparatus. The two-sided conveyance unit control unit 209 performs a sheet reversing and re-feeding operation inside the two-sided conveyance unit 130 according to an instruction from the engine control unit 202. Further, the two-sided conveyance unit control unit 209 sends an operation status of the two-sided conveyance unit 130 to the engine control unit 202.
An envelope feeder control unit 210 controls an operation of an envelope feeder which is detachably attached to the image forming apparatus. The envelope feeder control unit 210 drives and stops a driving system of the envelope feeder according to an instruction from the engine control unit 202. Further, the envelope feeder control unit 210 sends information about whether there is an envelope in the envelope feeder to the engine control unit 202.
FIG. 13 illustrates a schematic configuration of a conventional direct current voltage application circuit that is usable in a laser beam printer. Hereinafter, a direct voltage will be referred to as a DC bias.
Referring to FIG. 13, a DC bias application circuit 501 includes a voltage setting circuit unit 502, a transformer driving circuit unit 503, a high-voltage transformer 504, and a feedback circuit unit 505. The voltage setting circuit unit 502 can change a setting value according to a pulse width modulation (PWM) signal and set a voltage to be applied to a load. The high-voltage transformer 504 serves as a unit for generating a high voltage. The transformer driving circuit unit 503 is a circuit for driving the high-voltage transformer 504. The feedback circuit unit 505 detects a voltage value applied on a load using a resistance R81, and feeds back the detected voltage value to the voltage setting circuit unit 502 in an analog value. A voltage is controlled to be applied on the load at a constant value based on the fed back analog value. Above-described charging roller 106 is an example of the load. Here, Vcc is a power source voltage.
The above-described circuit configuration enables applying of a constant voltage to a load by controlling the voltage to be applied to a load. Japanese Patent Application Laid-Open No. 6-3932 discusses a technique related to such a circuit configuration. A configuration of a DC bias application circuit discussed in Japanese Patent Application Laid-Open No. 6-3932 can control a voltage value applied to a load to be constant. However, since there is no configuration to detect a current value flowing in the load, the applied voltage cannot be accurately output according to the current flowing in the load.
Moreover, there is a demand to switch control between the above-described constant voltage control which controls a voltage applied to a load to be constant according to a load status (i.e., detected current value), and constant current control which controls a current flowing in a load to be constant. Conventionally, in a case where the control is to be switched between the constant voltage control and the constant current control, a constant voltage control circuit and a constant current control circuit are separately provided (for example, refer to Japanese Patent Application Laid-Open No. 10-32979 and Japanese Patent Application Laid-Open No. 9-179383).
Therefore, conventionally, two control circuits are separately provided for switching the control between the constant voltage control and the constant current control as described above. Therefore, such configuration increases circuit sizes and cost for configuring circuits.
Further, if a plurality of control circuits is provided, switching operation may be required in consideration of each circuit operation status, so that switching between circuits takes time. The longer the time for the switching operation, the more the time to output a target voltage on a load, so that the time for the switching operation increases an entire operation time of the apparatus.